Using foreach with index in C#

Just a quick tip today!

for and foreach loops are among the most useful constructs in a C# developer’s toolbox. To iterate a collection, foreach is, in my opinion, more convenient than for in most cases. It works with all collection types, including those that are not indexable such as IEnumerable<T>, and doesn’t require to access the current element by its index.

But sometimes, you do need the index of the current item; this usually leads to one of these patterns:

// foreach with a "manual" index
int index = 0;
foreach (var item in collection)
{
    DoSomething(item, index);
    index++;
}

// normal for loop
for (int index = 0; index < collection.Count; index++)
{
    var item = collection[index];
    DoSomething(item, index);
}

It’s something that has always annoyed me; couldn’t we have the benefits of both foreach and for? It turns out that there’s a simple solution, using Linq and tuples. Just write an extension method like this:

using System.Linq;
...

public static IEnumerable<(T item, int index)> WithIndex<T>(this IEnumerable<T> source)
{
    return source.Select((item, index) => (item, index));
}

And now you can do this:

foreach (var (item, index) in collection.WithIndex())
{
    DoSomething(item, index);
}

I hope you find this useful!